The present invention relates generally to communication devices, and more particularly, to a data format used by such devices to maximize the amount of information that can be conveyed by the data format.
An example of a communication device is a remotely powered electronic device and related system for powering up and receiving stored information from such device. For example, U.S. Pat. No. 4,818,855, issued to Mongeon et al., entitled, Identification System, discloses a remotely powered identification device which derives power from a remote source via one of an electric field or a magnetic field and which transmits stored information back to the source via the other of the electric field or magnetic field. Similarly, U.S. Pat. No. 5,009,227, issued to Geiszler et al., entitled, Proximity Detecting Apparatus, discloses a remotely powered device which uses electromagnetic coupling to derive power from a remote source and which uses both electromagnetic and electrostatic coupling to transmit stored data back to the source.
An example of a remotely powered electronic device and related system is a proximity identification system. In this system, an interrogator, typically located in a fixed installation, transmits an interrogating signal to activate a portable transponder type device (e.g., a tag). The tag is ordinarily carried by a person, an animal or an object, which is in the vicinity or proximity of the interrogator. The signals from the interrogator causes the transponder to continuously produce and transmit a coded identifying signal to a receiver conventionally located at the same location as the interrogator. The receiver, upon detection of a valid identifying code from the transponder, activates some system functions, e.g., to provide access to a controlled area or to keep track of the person, animal or object.
The identifying code conforms with a predetermined data format so that the receiver can determine its validity. Several formats have been suggested and used. For example, U.S. Pat. No. 5,382,952, issued to Miller and entitled Transponder for Proximity Identification System, discloses a 64-bit data format (i.e., each frame contains 64 bits). This format provides for a 32-bit synchronization field, one start bit, and an optional parity bit. The rest of the bits are variable data bits which can be used to assign distinct identifying codes to different tags. According to Miller, the 32-bit synchronization field is required for correct decoding. This data format is not efficient because less than half the frame can be used to store identifying codes.
The number of distinct identifying codes associated with a data format increases with the number of variable data bits. Specifically, the maximum number of allowable distinct identifying codes is equal to 2.sup.n, where n is the number of variable data bits. Thus, in order to increase the number of identifying codes, it is desirable to have as many variable data bits as possible.
It should be appreciated that the variable data bits can be used to convey many types of information in addition to identifying codes. Also, the data format can be used in many communication devices in addition to proximity identification systems.